One known way of controlling the lifting and traversing motions of a crane is to use push-button controllers to give control commands. When the motor driving the crane is powered via a converter, the push-button controllers are used to give the converter a motor speed reference signal, on the basis of which the converter adjusts the motor speed to the desired value. A known system for controlling the motor speed, using push-button controllers, is presented in publication GB A 2171863 (Int. Cl. H02P 7/42), which provides, for each direction of motion, a two-position push-button to determine both the direction and speed of the motion. A first push-button position determines the direction of rotation of the motor and causes acceleration of the motor to a crawling speed (or, when the motor is already running, the maintenance of the speed already reached), while a second position causes the motor to accelerate to the maximum allowed speed. When the button is completely released, the motor speed is reduced at a constant rate of deceleration.
When this type of two-position push-button control is used in connection with a traversing motor, its speed of rotation is accelerated to the maximum in about three seconds. This is generally sufficient for the operator to be able to adjust the motor speed by returning the button to the first position to maintain a desired speed. In the case of the lifting motions, however, the acceleration and deceleration times are shorter, typically 0.5 to 1.5 s. In a practical situation, it is difficult to accurately adjust the speed because it is determined on the basis of a short push-button activation time. In the case of the shortest, 0.5 s acceleration times, this means that if the motor is to be operated at 1/3 of its maximum speed, for example, the push-button will have to be held at the 2nd step for about 0.2 s and then returned to the 1st step. This time interval is too short to allow the operator to have a direct feel of the speed, in direct contrast to the so-called joystick control, where the joystick position is proportional to the speed.